Liquid phase diffusion bonding is a bonding method whereby an alloy in the form of a foil, powder plating and having a eutectic composition with a lower melting point than the materials to be bonded is situated and compressed between the materials to be bonded, and the joint is heated to a temperature just above the liquidus of the insert alloy (hereunder referred to as "insert metal") for melting followed by isothermal solidification, and it is considered to be one type of the solid phase bonding methods. Because liquid phase diffusion bonding allows bonding to be accomplished with relatively low pressure application, it has been conventionally used for bonding which requires absolutely minimal residual stress and deformation by bonding and has mainly been applied for high-alloy steel and heat-resistant steel for which simultaneous welding is difficult, and for bonding between these and carbon steel.
Liquid phase diffusion bonding is most commonly employed for bonding of materials containing at least 0.50% Cr as the alloy composition. Cr-containing materials are characterized by forming dense Cr oxide (usually Cr.sub.2 O.sub.3) films on their surfaces, and thus exhibiting excellent oxidation resistance and corrosion resistance. Consequently, the bonding heat also naturally causes formation of oxide films on the bonding surface, and this inhibits wetting of the molten insert metal and considerably interferes with diffusion of atoms which is essential for bonding, so that it has been difficult to obtain satisfactory joints.
For this reasons all such conventional attempts, such as seen in Japanese Unexamined Patent Publications No. 53-81458, No. 62-34685 and No. 62-227595, have tended to entail notable increases in bonding costs, since the vacuum, inertness and reducing property of the bonding atmosphere must be maintained for the liquid phase diffusion bonding.
The present inventors have conducted much research aimed at providing liquid phase diffusion bonding to be employed as a means of bonding stainless steel, high nickel-based alloys, heat resistant alloy steels and other related alloy steels, whereby the liquid phase diffusion bonding can be achieved even when an oxide film is produced on the bonded material surface in air, to give a satisfactory joint in a short time and at a reduced bonding cost, and as a result we have found that such liquid phase diffusion bonding is possible even in an oxidizing atmosphere such as air by using an insert metal containing P or B, or P and B, as the diffusing elements, containing V at 0.1-20.0 atomic percent and having an increased amount of Si.
In other words, it had already been found that while V is the element which increases the melting point of the insert metal, an insert metal with very excellent bonding properties can be obtained by appropriately adjusting the other elements (entirely Si according to the invention), and this is employed for liquid diffusion bonding of primarily stainless steel, high nickel-based alloys, heat resistant alloy steels and these alloy steels with carbon steel, such as already disclosed in Japanese Unexamined Patent Publications No. 2-151377, No. 2-151378, No. 2-185940, No. 7-268521, No. 7-276066, etc. Liquid phase diffusion bonding alloy foil has been proposed which allows bonding of a Ni base containing P or B, or P and B, as the diffusing elements and containing V and Si, in an oxidizing atmosphere.
However, although the aforementioned inventions are all foils with high bonding properties containing large amounts of Si for the purpose of lowering the melting point of the insert metal, they are often unsuitable as bonding alloy foils in cases which demand casting properties for the foils and relatively high toughness for the bonded materials.
Specifically, it often occurs that the elasticity of the foil is reduced rendering it very brittle, or that the Si diffuses into the matrix after liquid phase diffusion bonding, resulting in an unwanted hardness increase and a considerably lower toughness in the so-called weld heat-affected zone.